Rechargeable batteries in cellular phones and other portable communication devices, such as NiCd, nickel-metal hydride (NiMH), Lithium-ion, and Lithium-Polymer batteries and Super Capacitors, can be recharged with household alternating current (AC) power coupled through a voltage reduction transformer, an alternating-to-direct current converter, and appropriate battery monitoring and charging circuits. They can also be recharged with a 12-volt cigarette lighter socket provided in an automobile coupled through a DC voltage reduction circuit and appropriate battery monitoring and charging circuits. However, in both cases, the portable communication device must be plugged into the household AC power source or into the automobile power source, limiting the mobility of the communication device.
Recently, wireless charging has become available for rechargeable batteries in cellular phones and other portable communication devices, using contact-less electromagnetic induction. A power source circuit in a wireless charging device drives a resonant frequency circuit that produces a source alternating current in a frequency range for example between 50 kHz and 20 MHz, which is driven through a transmitting coil in the charging device. The alternating magnetic field produced by the transmitting coil inductively couples with a corresponding receiving coil in the cellular phone or other portable communication device, thereby producing a corresponding induced alternating current that drives a circuit at its resonant frequency in the range for example between 50 kHz and 20 MHz to produce an output AC voltage. A conversion circuit in the cellular phone or other portable communication device, uses a transformer to adjust the output AC voltage, an alternating-to-direct current converter, and appropriate battery monitoring and charging circuits to produce an appropriate DC charging voltage for the rechargeable battery.
Large sized wireless charging pads have become available to charge rechargeable batteries in multiple portable communication devices, high powered hand tools, domestic appliances, or garden tools using contact-less electromagnetic induction. Wireless charging pads are generally shaped as a flat plate and typically have an active charging surface approximately the size of a sheet of typing paper. Other shapes for the charging pad may not be flat, but instead shaped to conform to particularly shaped user devices to be charged, for example a charger shaped as a wall-mounted holder for a garden tool. Wireless charging pads use multiple transmitting coils or a single large transmitting coil to distribute their magnetic flux over the active charging surface.